A well known measure to decrease black smoke discharged from a diesel engine of a vehicle is to use a diesel particulate filter (hereinafter referred to as DPF) which traps particulate matter contained in the exhaust gas of the diesel engine.
When the amount of trapped particulate matter reaches a certain level, the DPF performs regeneration by burning the trapped particulate matter, and thus becomes able to trap particulate matter again. In order to burn the particulate matter, the temperature of the exhaust gas is raised by fuel injection control, or the temperature of the DPF is raised using a heater.